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Cooling or heating agitated liquid in vessels is a basic technological operation in the

chemical, biochemical,pharmaceutical, food and processing industries. The cooling or

heating rate depends on how the heat is supplied or removed, the mixing intensity and

many other parameters. Good knowledge of all parameters is important for the design of

real equipment, e.g. fermentors for transforming biomass to biogas. The heat transferrate between tube baffle and an agitated liquid depends on many parameters, e.g. the

geometry, the agitated liquid properties, and the mixing intensity, which is influenced by

the type of agitator and its rotation rate.

Agitation is forcing a fluid by mechanical means to flow in a circulatory or other pattern

inside a vessel.

Aims of agitating fluids are mixing of two miscible liquids, dissolving solids in liquids, dispersing

a gas in a liquid as fine bubbles, suspending of fine solid particles in a liquid. The liquids are

generally agitated in cylindrical vessels with an impeller mounted on a shaft driven by a in

liquids,an electric motor. It is necessary to cool or heat the contents of the vessel during

agitation. This is done by heat transfer surfaces, which may be in the form of cooling or heating

jackets in the wall of the vessel or coils of pipes immersed in the liquids. In this experiment, the

heating jacket is used.

In the experiment; when the agitation speeds are N1=32 rpm, N2=64 rpm, N3=96 rpm, the

overall heat transfer coefficients and waterside heat transfer coefficients are calculated.

OBJECTS

The object of the experiment is to investigate the effect of the agitation speed on the

overall heat transfer coefficient and the water side heat transfer coefficient.

THEORY

The liquids are generally agitated in cylindrical vessels with an impeller mounted on a shaft

driven by an electric motor which has an adjustable speed mechanism. The flow patterns in an

agitated tank depend upon the fluid properties, the geometry of the tank, the types of the baffles in

the tank, and the agitator itself. The presence or absence of turbulence can be correlated with the

impeller Reynolds number:

Re= (DaN) /

Where, Da is impeller diameter, m; is fluid density, kg/m3, and is fluid viscosity,

kg/m.s; N is rotational speed, rev/s.

The agitated vessel has four propellers mounted on the same axis. Each propeller has two

wings. But only two of the propellers will be placed below the jacket vessel. (i.e. water level) in this

experiment. Power consumption is related to fluid density, fluid viscosity, rotational speed and

impeller diameter by plots of Power number versus Reynolds number. The power number (Np) is;

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NP=P/(N3Da

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Where, P is power, J/s.

It is necessary to cool or heat the contents of the vessel during agitation. This is done by heat

transfer surfaces, which may be in the form of cooling or heating jackets in the wall of the vessel orcoils of pipes immersed in the liquids. When heating, the fluid entering is often steam, which

condenses inside the jacket and leaves at the bottom. Correlations for the heat transfer coefficient from

the agitated Newtonian liquids inside the vessel to the jacket walls of the vessel have the following

form:

hDt/k = a(Re)b

(Pr)1/3(/w)

m

where, h is the heat transfer coefficient for the agitated liquid to the inner wall,

W/m2.K; Dt is the inside diameter of the tank, m; k is termal conductivity of fluid, W/m.K; Cp

is specific heat of the fluid, J/kg.K; w is liquid viscosity at the wall temperature, kg/m.s ; a, band m are emprical constants.

PROCEDURE

The vessel was filled with 50 L of water by a valve. The steam pressure should be constant at

1,2 bar.

Then the speed of the mixer was adjusted by the adjustment of the switch on the motor but

the rotation speed was not known so we calculated the speed with the help of chronometer. We

counted the rotation number of the shaft per second the first speed we adjusted was 32 rev/min.

We waited for the temperature to reach at 34C. After 34C, we recorded the temperature

values every 30 seconds. This was done until 84C was obtained. When 84C was reached we close

the steam valve and opened the outlet valve to drain the vessel filled with a 50 L of water. The steam

in the jacket cooled in a short time and condenses. When the steam valve is opened again, the

condensed water penetrates out through the outlet and replaces with steam again.

The same procedure was repeated for three different agitation speeds which were 64

rev/min, 96 rev/min respectively. During of three tests all temperatures between 34C and 84C were

recorded at 30 seconds period.